The present invention relates to an exposure control apparatus for an electronic still camera, especially a camera of a type having a TTL (through the lens) light measuring system.
In electronic still cameras, still video images formed on a solid state imaging device such as a CCD image sensor are photo-electrically converted into electric video signals, which are recorded on a still video floppy disc. Because a solid state imaging device generally has a narrow dynamic range, it is necessary to control the exposure precisely.
In general, the exposure of an electronic still camera is controlled by setting a shutter speed and an aperture size according to an automatic exposure control program. Though it is possible to use a mechanical shutter, it is inexpensive and preferable to use a sensor shutter which adjusts a charge storage time of the solid state imaging device. Furthermore, it is desirable to measure the subject brightness through a TTL light measuring system so as to control the exposure properly. Therefore, there are conventional still cameras which employ a TTL light measuring system and a sensor shutter.
As is known in the art, the solid state imaging device always is illuminated, and the charge storage phase and the read out phase of the solid state imaging device are repeated periodically so long as a power switch is turned on. Therefore, unnecessary light entering at a time other than an appropriate main charge storage time, that is, a necessary exposure time for picking up an image, may produce a noise charge in the charge storage portion or the charge transfer channel of the solid state imaging device. If such a noise charge leaks to the charge transfer channel, it will cause a smear noise in the produced image, lowering the quality of the image remarkably.
In order to reduce the effect of such noise charges, unnecessary charges are swept out of the solid state imaging device before the charge is stored. However, unnecessary light entering immediately after the charge storage time may still cause noise charges.